to allow for creating and securing frames in advance; Create and secure frames in advance when sending bursts; Do neither recreate nor resecure frames that come from phase
The CCA-threshold now defaults to -46 which give better simulation
results and typically also better experimental results.
This adjustment is also needed due to commit 0a13f99 in mspsim. As
promised in https://github.com/mspsim/mspsim/pull/18 it broke the
regression tests.
